UNVIS(3) MachTen Programmer’s Manual UNVIS(3)

NAME
unvis, strunvis - decode a visual representation of characters

SYNOPSIS
#include <vis.h>

int
unvis(u_char *cp, u_char c, int *astate, int flag)

int
strunvis(char *dst, char *src)

DESCRIPTION
The unvis() and strunvis() functions are used to decode a visual repre-
sentation of characters, as produced by the vis(3) function, back into
the original form. Unvis is called with successive characters in c until
a valid sequence is recognized, at which time the decoded character is
available at the character pointed to by cp. Strunvis decodes the charac-
ters pointed to by src into the buffer pointed to by dst.

The strunvis() function simply copies src to dst, decoding any escape se-
quences along the way, and returns the number of characters placed into
dst, or -1 if an invalid escape sequence was detected. The size of dst
should be equal to the size of src (that is, no expansion takes place
during decoding).

The unvis() function implements a state machine that can be used to de-
code an arbitrary stream of bytes. All state associated with the bytes
being decoded is stored outside the unvis() function (that is, a pointer
to the state is passed in), so calls decoding different streams can be
freely intermixed. To start decoding a stream of bytes, first initialize
an integer to zero. Call unvis() with each successive byte, along with a
pointer to this integer, and a pointer to a destination character. The
unvis function has several return codes that must be handled properly.
They are:

0 (zero) Another character is necessary; nothing has been recog-
nized yet.

UNVIS_VALID A valid character has been recognized and is available
at the location pointed to by cp.

UNVIS_VALIDPUSH A valid character has been recognized and is available
at the location pointed to by cp; however, the character
currently passed in should be passed in again.

UNVIS_NOCHAR A valid sequence was detected, but no character was pro-
duced. This return code is necessary to indicate a log-
ical break between characters.

UNVIS_SYNBAD An invalid escape sequence was detected, or the decoder
is in an unknown state. The decoder is placed into the
starting state.

When all bytes in the stream have been processed, call unvis() one more
time with flag set to UNVIS_END to extract any remaining character (the
character passed in is ignored).

The following code fragment illustrates a proper use of unvis().

int state = 0;
char out;

while ((ch = getchar()) != EOF) {
again:
switch(unvis(&out, ch, &state, 0)) {
case 0:
case UNVIS_NOCHAR:
break;
case UNVIS_VALID:
(void) putchar(out);
break;
case UNVIS_VALIDPUSH:
(void) putchar(out);
goto again;
case UNVIS_SYNBAD:
(void)fprintf(stderr, "bad sequence!0);
exit(1);
}
}
if (unvis(&out, (char)0, &state, UNVIS_END) == UNVIS_VALID)
(void) putchar(out);